1. Field of the Invention
The present invention relates to a cable connecting member which is directly connected to an apparatus and connects a power cable, such as a CV cable or an EP rubber insulating/EP rubber sheathed cable, and an electric power apparatus, such as a transformer or a switch, and to a cable connecting member used for connecting power cables, and, more particularly, the present invention relates to a cable connecting member for use in cold climates which is used at an environmental temperature including a low-temperature range, such as from 80° C. down to −40° C., preferably from 80° C. down to −60° C.
2. Description of the Related Art
Conventionally, a cable connecting member shown in FIG. 8, for example, is used in connecting a power cable and an electric power apparatus or connecting power cables.
FIG. 8 is a sectional view schematically showing the configuration of a conventional cable connecting member which is directly connected to an apparatus and is T-shaped (hereinafter a “directly-connected (T-shaped) cable connecting member”).
In FIG. 8, a directly-connected cable connecting member 800 has a rubber insulating tube 801 housing an end of a cable 850 and enhancing electrical insulation from the cable and a rubber spacer 803 inserted into an inner semiconducting layer 802 provided in the rubber insulating tube 801. Moreover, at a cable insertion-side end of the rubber spacer 803, an outer semiconducting layer 804 for alleviating electric field concentration is formed. The rubber spacer 803 is used as an adapter for compensating for a fit diameter difference when the inside diameter of the inner semiconducting layer 802 is larger than the outside diameter of an insulation layer 851 of the cable 850 used, or to make it possible to apply a common rubber insulating tube to several types of cables having different outside diameters. The rubber insulating tube 801, the inner semiconducting layer 802, and the rubber spacer 803 are formed of ethylene propylene rubber (hereinafter referred to simply as “EP rubber”), or the rubber insulating tube 801, the inner semiconducting layer 802, and the rubber spacer 803 are formed of silicone rubber. Incidentally, in FIG. 8, an outer semiconducting layer and a metal shielding layer of the cable, connection by a semiconducting fusion rubber tape or the like which electrically connects an outer semiconducting layer in the rubber spacer and the outer semiconducting layer of the cable, leading out of a grounding conductor, and the like, are not shown, and the description thereof will be omitted.
In the directly-connected cable connecting member configured as described above, when the rubber spacer 803 is fitted over an end of the insulation layer 851 of the cable 850 and the rubber spacer 803 is inserted into the rubber insulating tube 801 in which the inner semiconducting layer 802 is provided, the interface between the rubber spacer 803 and the rubber insulating tube 801 is held at a predetermined contact pressure by the rubber elasticity of the rubber insulating tube 801, whereby insulating characteristics are ensured. Likewise, insulating characteristics are ensured also at the interface between the insulation layer 851 of the cable 850 and the rubber spacer 803.
Here, in cold climates, the temperature of an environment in which a cable connecting member is placed sometimes decreases from room temperature to −30° C. or lower. In this case, the elongation modulus of EP rubber exhibits temperature dependence shown in FIG. 2, and shows a tendency to increase sharply at −30° C. or lower. Since the EP rubber tends to become hard with increasing elongation modulus of elasticity, the contact pressure at the interface with the rubber spacer decreases. When a current passing through the cable is small and a rise in the temperature of a conductor is small, the temperature of the cable connecting member decreases as follows. The temperature of the rubber insulating tube exposed to an external environment first decreases, and the temperatures of the rubber spacer, the insulation layer of the cable, the conductor, and the like, which are placed inside the rubber insulating tube eventually decrease with decreasing temperature of the rubber insulating tube. For example, when the EP rubber is almost completely hardened as a result of the temperature of the rubber insulating tube having decreased to −50° C. and the elongation modulus of the EP rubber having increased to a level which is three or more times as high as that at room temperature, the temperature inside the rubber spacer does not decrease with decreasing temperature of the rubber insulating tube and is sometimes higher than the temperature of the rubber insulating tube. At this time, as time passes, the temperature inside the rubber spacer also decreases to a temperature that is equal to that of the rubber insulating tube, and the EP rubber of the rubber spacer is also hardened almost completely. However, since the EP rubber of the rubber insulating tube is hardened and, while keeping the shape thereof, the temperature inside the rubber spacer further decreases, the outside diameter of the rubber spacer becomes smaller than the inside diameter of the rubber insulating tube observed when the rubber insulating tube was hardened, whereby a gap is formed at the interface between the rubber insulating tube and the rubber spacer. When this gap grows to several tens of micrometers or more, partial discharge occurs in this gap, which may produce a dielectric breakdown at a working voltage due to discharge degradation of the interface. Moreover, a gap is also formed at the interface between the rubber spacer and the insulation layer of the cable, which may produce a dielectric breakdown also at the interface between the rubber spacer and the insulation layer of the cable.
To solve this problem, a cable connecting member shown in FIG. 9 have been used. Another conventional directly-connected (T-shaped) cable connecting member is shown in FIG. 9. In FIG. 9, a cable connecting member 900 includes an insulating layer 901 formed of cross-linked silicone rubber, an inner semiconducting layer 902 formed of cross-linked silicone rubber, and an outer semiconducting layer 903 formed of cross-linked EP rubber. In this cable connecting member, a power cable terminal obtained by attaching a terminal to a conductor of a power cable is inserted into a cable terminal holder 904, and an apparatus terminal obtained by attaching a bushing to a conductor of an apparatus is inserted into an apparatus terminal holder 905. In this way, the power cable terminal and the conductor of the apparatus are mechanically connected (Japanese Laid-Open Patent Publication (Kokai) No. 2003-348744).
Even when the environmental temperature is −50° C., the silicone rubber does not show a tendency to become hard because an increase in its elongation modulus from that at room temperature to that at −50° C. is small (see FIG. 2), and has rubber elasticity which is equal to that at room temperature. Thus, a gap is not formed at the interface between the cable terminal holder 904 and a cable insulator until after the temperature inside the insulating layer 901 has decreased with decreasing temperature of the outer semiconducting layer 903, and a dielectric breakdown does not occur.
However, the problem of the technique proposed by Japanese Laid-Open Patent Publication (Kokai) No. 2003-348744 is that, since the insulating layer is formed in almost the entire region inside the outer semiconducting layer, and the mechanical strength of the silicone rubber is lower than that of the EP rubber, the insulating layer is susceptible to mechanical damage and is likely to cause a decrease in insulating performance. Moreover, the silicone rubber has high water absorption, causing a problem of a decrease in insulating performance in humid conditions such as when it is snowing or raining. Furthermore, since the outer semiconducting layer delimiting an insertion opening of the cable terminal holder is formed of EP rubber, it is difficult to apply a common rubber insulating tube to several types of cables having different outside diameters.